During the last year, we continued define the molecular events that regulate hematopoietic stem cell (HSC) quiescence, survival, self-renewal and, myeloid cell lineage commitment and differentiation. We have focused our efforts on transcription factors since they are essential for stem cell and lineage development and, are often deregulated during the development of leukemias and lymphomas. Hematopoietic transcription factors regulate the expression of hematopoietic growth factors (HGF), HGF receptors, other transcription factors, and lineage specific genes. In this regard, we have discovered that the helix loop helix transcription factor, Id1, is not expressed in normal HSC and common lymphoid progenitors (CLP), but is expressed in more committed common myeloid progenitor cells (CMP) There are four Id proteins (Id1-Id4), which function as dominant negative regulators of other helix loop helix transcription factors as the Id proteins lack the basic region required for DNA binding. We have discovered that HGF that drive myeloid development instruct HSC toward a myeloid versus an erythroid and lymphoid cell fatein vitro. These results have been confirmed by over expressing Id1 in HSC in vivoandin vitro. Furthermore, Id1 greatly enhances the proliferation and growth of CMP and GMP in vitro and in vivo suggesting that Id1 can enhance myeloid cell proliferation. We have we found that mice transplanted with Id1-infected bone marrow cells became moribund beginning at six to eight months. Histological analysis of moribund mice indicated that these mice had increased extramedullary hematopoiesis and hyper proliferation of myeloid cells, suggesting that these mice developed a myeloproliferative disease (MPD). BMC from these mice did not develop leukemia when transplanted into immune deficient mice which confirmed the MPD, however, HGF-dependent immortalized cell lines can be readily established from BMC of mice with MPDin vivo. These cell lines have an immunophenotype that is representative of normal CMP and GMP progenitor cells suggesting a hyper proliferation of this compartment in the late stages of the MPD. We have extended our analysis of Id gene function to evaluate Id2, which is expressed in hematopoietic cells, can also regulate cell fate decisions, and promote progenitor cell proliferationin vitroandin vivo. We have found that the highest levels of Id2 are expressed in megakaryocyte/erythroid progenitors (MEP) compared to HSC, CMP and GMP suggesting Id2 may regulate erythroid cell fate decisions. In this regard, over expression of Id2 in the EML stem cell line model inhibits myeloid and promotes erythroid development. In addition, Id2 also promotes macrophage and inhibits neutrophil differentiation suggesting that Id2 regulates cell fate decisions in more mature myeloid cells. Finally, over expression of Id2 in HSC promotes primitive progenitor cell proliferation and inhibits B cell development from multipotential progenitors. Thus, Id2 may regulate cell fate decisions in primitive and more committed hematopoietic progenitors. Mice that lack Id1 gene expression are viable and show no obvious defects. However, Id1 null mice show a significant reduction in bone marrow and spleen cellularity, increased percentages and cycling progenitor cells, and increased percentages of neutrophils and macrophages in the peripheral blood. These studies indicate that Id1 is required for regulating normal hematopoietic development. Future studies are designed to determine if these effects are intrinsic to the hematopoietic cells or to a micro environmental defect. In addition, we are currently investigating hematopoietic defects in Id2 null and, Id1/Id2 null mice as Id genes may be compensatory in vivo.